Prior work in the field of the present disclosure has taken place along three directions: (i) generally developing the concept of photonic nanojets for dielectric microspheres, microcylinders, and microfibers; (ii) generally developing the concept of photonic nanojets (via theoretical modeling) for microcomponents with arbitrary shapes, and (iii) generally developing a proposal for the large-scale integration of arrays of microspheres, microcylinders, and microfibers with FPAs for enhancing the sensitivity and AOV of imaging devices.
The concept of photonic nanojets was originally introduced and developed for focusing light by dielectric microspheres (or two-dimensional microcylinders) with refractive index contrast in the 1.4<n<2.0 range and with diameters in the 2λ<D<100λ range. Microspheres and microcylinders concentrate light due to a refraction effect. They essentially operate as focusing microlenses. Calculations performed have showed that the waist of a focused beam can be below the diffraction limit, reaching values ˜λ/3 in some cases. Many applications for photonic nanojets have been proposed, including polarization filters based on chains of microspheres and focusing single-mode and multimode microprobes. More recently, an application for photonic nanojets related to focusing electromagnetic energy into a photodiode has been proposed theoretically.
More recently, it has been proposed that microcomponents with shapes different from microspheres (or microcylinders or microfibers) can be also used for creating tightly focused beams. These proposals have been based on the theoretical modeling of light propagation through such structures. In contrast to microspheres and microcylinders, the concentration of light by such microcomponents is based not only upon the refraction of light (like a lens), but also upon waveguiding and/or light scattering effects. To illustrate this distinction from microspheres and microcylinders, one example of such a microcomponent is represented by a tapered waveguide where the concentration of light is provided by total internal reflection, as opposed to refraction. The following structures have been considered: cuboids, microcones, pyramids, and axicones. It has been demonstrated that photonic jets in such structures are characterized by a substantially increased optical intensity with respect to the incident radiation and possesses narrow beam width. The length of such photonic jets can reach several particle diameters, while the transverse width can have sub-diffraction-limited dimensions. The spatial parameters of the nanojets can be varied through variation of the structural parameters of the microcomponents. In these publications, however, the use of the microcomponents for increasing the sensitivity and AOV of FPAs was not considered. In addition, the potential advantages of such structures based on their manufacturability by standard fabrication methods (in comparison with microspheres, for example) were not reported.
The enhancement of light collection efficiency by the integration of individual pixels of mid-wavelength infrared (MWIR) FPAs with individual microspheres was recently demonstrated experimentally. The mechanism of this enhancement factor is based on the collection of light from the area determined by the size of the microsphere (which can be much larger than the size of the associated photodetector mesa) and the focusing of this beam into a photodetector mesa. This principle was demonstrated for individual MWIR detectors fabricated in a structure grown on a GaSb substrate with an InAs/InAsSb type-II strained-layer superlattice (SLS) as the active materials. The maximum increase of the sensitivity as compared to the case of photodetector mesas before integration with microspheres is determined by the geometrical factor, ˜(D/d)2, where D is the sphere diameter and d is the photodetector mesa diameter. An increase of sensitivity by up to ×100 in a broad range of wavelengths was demonstrated experimentally.
This work directly motivated a proposal for the large-scale integration of arrays of microspheres with FPAs for enhancing the sensitivity and AOV of imaging devices. Such sensitivity enhancement is explained above. The increase in AOV is introduced by comparison with standard microlens arrays that are currently used for focusing light into individual pixels. The increase in AOV is explained by the significantly shorter focal distances offered by microspheres as compared to that in the case of weaker lenses in standard microlens arrays. As a result, at oblique incidence, the beam focused by the microspheres experiences smaller lateral shift as compared to that in the case of standard microlenses. This explains the larger AOVs in the case of the use of microspheres.
The main problem associated with developing such FPAs integrated with microspheres is connected with the difficulties in obtaining defect-free large-scale arrays of dielectric microspheres directly at the surfaces of FPAs. The microspheres in a suspension or in the form of dry powder need to be manipulated individually and precisely to form such large-scale, perfectly ordered arrays. This is an extremely difficult task since the total number of spheres in such arrays can be in a 104-106 range. Traditionally, large-scale arrays of microspheres can be obtained by the techniques of directed self-assembly. Many modifications to directed self-assembly have been tried, including the self-assembly of microspheres on patterned electrodes by an applied electric field. Ordered 2-D arrays of 100-μm glass microspheres with a 1% defect rate have been obtained by this method. Another example is represented by a method using a shear force in the course of drying the suspension. One more example is represented by parallel manipulation of spheres using conventional or optoelectronic tweezers. In addition to wet fabrication techniques, the template self-assembly of microspheres into ordered 2-D arrays has been developed under dry conditions. However, these methods are not defect free and the error rate determined by the concentration of missing or interstitial spheres in such arrays usually cannot be made less than 1% even in most advanced methods, such as assembly on patterned electrodes by an applied electric field.
It has also been proposed to use a different method based on air suction through an array of microholes. This method has yielded very good results for relatively small arrays with the total number of spheres below 102-103. However, for larger arrays, it has still led to error rates on the order of 1%. The missing or displaced microspheres result in broken pixels after integration with FPAs and such concentration of defects is not acceptable in many imaging devices. Smaller concentrations of defects below ˜0.1% and, desirably, below ˜0.01% are required in high-quality imaging applications.
Another problem is how to align the microspheres with the photodetector mesas with about 1-2 μm accuracy required for efficient coupling of photonic jets into the photodetector mesas. This needs to be achieved for very large-scale photodetector arrays.
Thus, to summarize, although the integration with microspheres (or with microcylinders or microfibers in some cases) is a promising approach for developing relatively small-scale arrays, these technologies are not defect free, and they usually result in at least 1% pixel defect rate, which is not acceptable in many of the FPA applications.